Abstract

The arrival of humans and human-introduced species to Pacific islands resulted in significant, long-lasting transformations to local ecosystems. However, direct measurements of deep-time human effects can be difficult to quantify from archaeological datasets. Isotopically reconstructed diet of the Pacific rat (Rattus exulans), a commensal species introduced by humans during one of the most dramatic archaeological examples of past human migration and ecosystem alteration, can provide insight into resource availability and utilization within the human-commensal niche. Our results highlight significant long-term restructuring of nutrient flows through ecosystems resulting from human arrival and subsequent land use on three Polynesian islands. We also demonstrate that stable isotope analysis of often-ignored commensal taxa represents a tool for tracking human activities and ecosystem effects more broadly.The role of humans in shaping local ecosystems is an increasing focus of archaeological research, yet researchers often lack an appropriate means of measuring past anthropogenic effects on local food webs and nutrient cycling. Stable isotope analysis of commensal animals provides an effective proxy for local human environments because these species are closely associated with human activities without being under direct human management. Such species are thus central to nutrient flows across a range of socionatural environments and can provide insight into how they intersected and transformed over time. Here we measure and compare stable carbon and nitrogen isotope data from Pacific rat (Rattus exulans) skeletal remains across three Polynesian island systems [Mangareva, Ua Huka (Marquesas), and the Polynesian Outlier of Tikopia] during one of the most significant cases of human migration and commensal introduction in prehistory. The results demonstrate widespread δ15N declines across these islands that are associated with human land use, intensification, and faunal community restructuring. Local comparison of rat stable isotope data also tracks human activities and resource availability at the level of the settlement. Our results highlight the large-scale restructuring of nutrient flows in island ecosystems that resulted from human colonization and ecosystem engineering activities on Pacific islands. They also demonstrate that stable isotope analysis of often-ignored commensal taxa can provide a tool for tracking human land use and environmental effects.